The present invention relates to a wavelength conversion element and a manufacturing method thereof.
A quasi-phase matching (QPM) type wavelength conversion element (hereinafter also simply referred to as a “wavelength conversion element”) is attracting attention, which performs wavelength conversion by realizing quasi-phase matching using a periodically poled structure. The wavelength conversion element has an optical waveguide formed of a periodically poled structure (hereinafter also simply referred to as a “polarization inversion structure), which has a period that accords with a wavelength of fundamental light on which wavelength conversion is to be performed. The fundamental light is input into the optical waveguide and wavelength converted light, on which the wavelength conversion has been performed, is output.
The wavelength conversion element is mainly used in the field of optical communication, but is also starting to be used in the field of optical measurement, such as in electrooptic deflectors (refer to Japanese Patent Application Publication No. JP-A-10-83001, for example), in terahertz wave generation devices (refer to Japanese Patent Application Publication No. JP-A-2005-77470, for example) and in optical modulators (refer to Japanese Patent Application Publication No. JP-A-11-174390, for example).
In any application, a highly efficient wavelength conversion performance is required of the wavelength conversion element. For that reason, it is effective to increase an overlap of electric field distributions of the fundamental light and the wavelength converted light. However, due to asymmetry of a refractive index profile of the optical waveguide, it has been difficult to improve wavelength conversion efficiency of the wavelength conversion element. Hereinafter, this point will be explained in more detail.
For example, the optical waveguide of the wavelength conversion element is obtained by forming, using a known proton exchange method or a Ti diffusion method, a high refractive index region on a ferroelectric crystal substrate that has a polarization inversion structure formed in accordance with a known method. In the optical waveguide (hereinafter also referred to as a “diffusion type optical waveguide”) formed in this manner, as protons or titanium are diffused from a substrate surface, dependent on a density of these materials, the refractive index gradually diminishes as the depth increases. In the optical waveguide having this type of refractive index profile, as the fundamental light and the wavelength converted light have significantly different electric field distributions, it has not been possible to increase wavelength conversion efficiency.
Further, as another method to form the optical waveguide, a technology exists in which a ferroelectric crystal substrate, which is bonded onto a low refractive index substrate that is a base and which has a polarization inversion structure, is diced in straight lines such that it has a ridge shape in cross section (refer to M. Iwai et al., “High-power blue generation from a periodically poled MgO: LiNbO3 ridge-type waveguide by frequency doubling of a diode end-pumped Nd: Y3Al5O12 laser”, Applied Physics Letters, Vol. 83, No. 18, p. 3659-3661, 3 Nov. 2003, for example).